A known optical transmission system includes an interferometer and a source of optical signals. The interferometer comprises a four port optical coupler having first and second input ports and first and second output ports, an optical coupling means coupling the first and second output ports and including an optical non-linearity, and an optical amplifier. The source of optical signals is coupled to the first input port of the interferometer.
An optical input signal coupled to an input port of such an interferometer is split into two portions by the optical coupler, which portions counter-propagate round the coupling means, for example an optical fibre loop, to return to, and recombine at, the coupler. For a symmetric coupler, the optical path along the coupling means is the same for the two portions. So, for a 50:50 coupler and a symmetrically positioned amplifier, the portions recombine such that the input signal emerges from the port to which it was originally input. The input signal is said to be "reflected" by the interferometer. For this reason, this configuration is often described as a loop mirror, the "loop" being the optical coupling means.
The specification of our co-pending International patent application, publication number WO 88/02875, describes an interferometer having a non-linear optical coupling means, namely a silica optical fibre loop, in which the symmetry of the two counter-propagating directions along the coupling means is broken to provide a differential non-linear effect (and so is called a non-linear optical loop mirror or NOLM). This can be achieved in various ways. For example, a non-50:50 coupler can be used. In this case, the intensities of the signal portions coupled into the ends of the waveguide loop are not equal. When the input signal is of sufficient intensity, the signal portions propagating in opposite directions around the waveguide experience different refractive indices. This results in the two counter-propagating signal portions experiencing different phase shifts, so that, when the signals return to the coupling means, they have an intensity-dependent relative phase shift. The intensity dependence of the relative phase shift results in a device whose output at an input port is, as is well known, an oscillatory function of the intensity of the input signal. Any signal exiting the second input port (that is to say the port to which the input signal is not coupled) is said to be "transmitted" by the interferometer.
A further way of breaking the symmetry of a NOLM is discussed in an article entitled "Nonlinear Amplifying Loop Mirror", by N. E. Fermann, F. Haberl, M. Hoffer, and H. Hochreiter, Opt. Lett., 15, p. 752, (1990), in which an amplifier is placed asymmetrically within the non-linear loop close to one of the output porks of the optical coupler, which in this case is a 50:50 coupler. Such an arrangement improves the performance of the conventional NOLM, in particular by better exploitation of the waveguide loop non-linearity, as it can be accessed by a smaller input signal. The experiments described in the Fermann et al article were carried out at low signal powers, and at repetition rates which did not saturate the gain of the amplifier. It was there noted, however, that amplifier saturation leads to a reduction in the overall gain of the device although, owing to the low pulse fluences, amplifier saturation in each individual pulse could still be neglected. Such a device is called a non-linear amplifying loop mirror (NALM).
Such NOLMs and NALMs can provide pulse shaping in optical transmission systems, and in particular provide pedestal suppression. Thus, these devices have the potential for the suppression of inter-pulse radiation, and for filtering bits in long-distance, all-optical communications systems. Such applications are discussed in an article entitled "Pulse Shaping, Compression, and Pedestal Suppression employing a Non-Linear Optical Loop Mirror" by K. Smith, N. J. Doran, and P. G. J. Wigley, Opt. Lett., 15, p. 1294 (1990).
A NALM could provide amplification in addition to such pulse shaping in an all-optical communications system. However, if the NALM has an oscillatory output, the intensity of the input signal must be relatively constant in order to avoid reflection by the loop mirror.
One way of removing the oscillatory output of a NALM is disclosed in an article titled "All-Optical Pulse Compression Using Amplifying Sagnac Loop" by R. A. Betts, S. J. Frisken, C. A. Telford and P. S. Atherton in Electronics Letters Vol 27 No. 10 (May 9, 1991). In their apparatus, the non-linear element in the loop is a semiconductor laser amplifier (SLA). This provides a saturating non-linearity which suppresses the oscillatory behaviour to provide a linear but rising response. A NALM which provides an approximately constant output would be more attractive for use in optical communications systems.